Add TypeScript types for types

**Problem**
When working with auth, the error messages could be more descriptive.

**Suggestion**
Improve error handling and add detailed error messages.

**Context**
This would help with debugging and maintenance.

No files, tests, or entry points are named. Start by locating the authentication error-handling and TypeScript type definitions, then clarify which types and error cases are in scope. The issue needs explicit acceptance criteria before a newcomer can tell when it is done.

Written by the indexing model from the issue text.